As metalworking processes become more complex and sophisticated and operate at higher pressures and speeds, increased demands are placed on lubricants to withstand the severe forces generated by hostile environments of the metalworking processes. Typical metalworking processes involve elastic or plastic deformation or cold working of metals. Examples of such metalworking processes include cold rolling of steel or aluminum sheets or foils, stamping, drawing, ironing, cutting, grinding, broaching, drilling and machining of metallic materials. The metallic materials from which the metalworking apparatus and articles to be fabricated are made include steel, cast iron, and ferrous alloys, as well as aluminum alloys and other non-ferrous alloys including such components as titanium, magnesium, copper, tin and brass.
Desirable lubricants not only reduce the coefficient of sliding friction between contacting metallic surfaces, but, among other attributes, control the temperature of the metalworking components and article during the metalworking process.
Lubricants are generally any liquid or solid which when used alone or with other components of a composition reduce friction between metallic parts and facilitate metal removal. Lubricity is the ability of a lubricant material to effectively reduce friction and wear of two metallic surfaces in processes involving elastic or bulk plastic deformation of one or both of the metallic surfaces. Lubricity of a lubricant is reflected in the degree of smoothness of surface finish of a product of a metalworking process following deformation, as well as the ability to control the temperature of the metalworking components and article during deformation and strain distribution in the metal product formed during the metalworking process.
Presently, conventional synthetic ester lubricants, such as polyol esters, and other natural fats are used as lubricating fluids for metalworking operations. U.S. Pat. No. 3,526,596 discloses that polyol esters of fatty acids having twelve to twenty-two carbon atoms, preferably polyols having two to twelve hydroxy groups and glycols having two to forty carbon atoms, are useful as lubricants in metalworking operations.
U.S. Pat. No. 3,681,440 discloses lubricants of esters derived from aliphatic or aromatic monocarboxylic acids and dineoalkyl ethers having a tetrahydroxy functionality.
U.S. Pat. No. 4,178,261 discloses a base lubricating oil of a carboxylic acid ester formed by reacting 6-cyclohexylhexanoic acid, optionally in combination with an aliphatic monocarboxylic acid having 4 to 20 carbon atoms, with a polyhydric alcohol.
U.S. Pat. No. 4,871,476 discloses a synthetic lubricating fluid for power transmissions which comprises (a) an ester or its derivative of cyclohexanol and cyclohexanecarboxylic acid or its derivative; and (b) 1% to 70% by weight of a branched poly-.alpha.-olefin. The ester has a traction coefficient about 5% to 7% higher than those of commercially available traction-based oils having a viscosity in the same range.
U.S. Pat. No 4,786,427 discloses lubricants of ester compounds for use in traction drives. U.S. Pat. No. 4,978,468 also discloses a traction fluid, the fluid comprising (1) at least one ester compound or its derivative containing a cyclohexyl or alkyl substituted cyclohexyl group joined to a linear chain hydrocarbon group by an ester group; and (2) 0.1% to 95% by weight of at least one polymer selected from hydrocarbon polymers, such as polyolefins and hydrogenation polyolefins, and polyesters such as polyacrylates and polymethacrylates. The ester components exhibit high traction coefficients in excess of 0.075.
In metalworking operations, as opposed to traction drives, it is desirable to have a lubricant possessing low full film traction and sliding friction coefficients. Full film lubrication is defined herein to mean a condition of lubrication in which the film thickness of the lubricant is appreciably greater than that required to cover the surface asperities of the metallic surface when subjected to the operating load, so that the effect of the surface asperities is not noticeable. See Organization For Economic Cooperation and Development, Friction, Wear and Lubrication Glossary at page 61 (Paris, 1969). In regions where full film lubrication cannot be achieved, it is desirable that such a lubricant have a low full film traction coefficient under hydrodynamic lubricating conditions and a low friction coefficient under boundary lubricating conditions.
Lubricants for use in metalworking operations must be capable of withstanding the high shear forces, temperatures and pressures encountered in such operations and provide a film of sufficient thickness to protect contacting metallic surfaces. Such a lubricant should resist viscosity changes during the metalworking process and possess a shear yield stress lower than that of the metallic surfaces. Ductility and mobility under severe operating conditions and a high affinity for metallic surfaces for inhibiting metal-to-metal contact between the surfaces is also desirable.
It is desirable for a metalworking lubricant to have not only high pressure rheological and low traction properties but also to exhibit hydrolytic stability. Typical known synthetic esters, primarily of the neopentyl type, suffer from hydrolytic instability. High stability to oxidative or hydrolytic breakdown is particularly important, because thermal or hydrolytic decomposition can result in the lubricant itself becoming corrosive and/or volatile, as well as generally less effective. Also, as the lubricant degrades, sludge and other decomposition products may deposit or form on the surfaces of the metallic components and articles and adversely affect the metalworking process.
Other drawbacks of lubricants suffering from thermal and hydrolytic instability include discoloration of the work product and degradation in the quality of the work product due to inconsistencies in the machining process because of viscosity changes or coating ability of the lubricant. Thus, it is desirable to have a lubricant which possesses not only a low coefficient of sliding friction, but also one which is hydrolytically stable to counteract water added to emulsify the lubricant composition and water used to cool the metal surfaces during the metalworking processes.